J. González-Hernández

Critical volume fraction of crystallinity for conductivity percolation in phosphorus‐doped Si:F:H alloys

We have used Raman scattering to deduce the volume fraction of crystallinity for the highly phosphorus‐doped glow‐discharge Si:F:H alloys. The measured critical volume fraction at the onset of rapidly increased conduction in this two‐phase system of microcrystallites embedded in an amorphous matrix is 0.18. This value coincides with the theoretical percolation limit and serves to explain the conduction process in these two‐phase materials which are useful as contacts in amorphous solar cells.

Raman, transmission electron microscopy, and conductivity measurements in molecular beam deposited microcrystalline Si and Ge: A comparative study

The structure of molecular beam deposited microcrystalline silicon and germanium films prepared under different growth conditions has been analyzed by conventional transmission electron microscope (TEM) and Raman scattering (RS) and compared to electrical conductivity experiments. The TEM measurements yield an average grain size L0 ranging from 200 A to 1.5 μm. On the other hand, the line shape of the RS is determined by a mean free path related to the average separation l between defects (or impurities), where l≤150 A. We find in a number of cases that the electrical conductivity is determined by l rather than L0. This experiment demonstrates the significance of RS as a structural characterization method when used in conjunction with other techniques such as TEM.

Optical properties of dense thin‐film Si and Ge prepared by ion‐beam sputtering

The optical properties of ion‐beam‐sputtered Si and Ge have been measured using spectroscopic ellipsometry over the range of 2.5–5.0 eV. Measurements have been performed on films prepared at different substrate temperatures (Ts) . An analysis of the spectroscopic ellipsometry data using the Bruggeman effective medium approximation reveals that very dense polycrystalline Si ( p‐Si) and Ge ( p‐Ge) films are obtained (without postdeposition heat treatment) for Ts ≥350 °C and Ts ≥200 °C, respectively. Maintaining a sufficiently high Ar beam voltage in the sputtering process is shown to be beneficial to the densification of p‐Si. Discrepancies observed between the structure of the p‐Si deduced from Raman and ellipsometry spectra are also addressed. The ellipsometry data are effective in detecting heterogeneity possibly due to surface roughness for the p‐Ge.

Determination of the energy barrier for structural relaxation in amorphous Si and Ge by Raman scattering

Abstract From the measured Raman linewidth of the transverse optic phonon for annealed amorphous Si and Ge we have, for the first time, determined an activation energy for the metastable state of the amorphous phase. In addition, we have evaluated the root mean square bond angle deviations from parameters for crystalline Si and Ge describing the Raman frequency shifts due to bond angle changes.

Nucleation and growth rate of a‐Si alloys

From fitting the crystallization rate to an Arrhenius rate expression with a single activation energy we found that it is possible to distinguish a purely amorphous silicon sample from one having trace crystallinity. Whenever microcrystallites are present whether from heavy P doping or heat treatment, the ‘‘activation energy’’ is approximately 1 eV, whereas purely amorphous silicon has an activation energy of 3–4 eV. In other words, the exponent on time in the Avrami expression is different for the nucleation and growth processes.

Order Parameters in a -Si Systems

THE AMORPHOUS-CRYSTALLINE transition in a-Si and a-Ge has been studied for many years [1,2]. However, unlike electrical and optical properties, no substantial modifications on the RDF (radial distribution function) for the amorphous phases have been reported for different methods of preparation or annealing conditions short of crystallization. More recently, Barna et al. [3] have found a small shift of the second peak in the RDF towards smaller r, and a more pronounced peak at 5 A for the a-Si: H prepared by glow discharge technique. These authors concluded that a higher degree of local order is present in the glow discharge a-Si: H although they stated that there exist only minor differences in amorphous silicon samples with different preparations. Earlier Raman measurements [4] have not been shown to be sensitive to preparation conditions, however, recent results indicated that Raman measurements can indeed differentiate various types of amorphous silicon samples. Tsu et al.

OPTICAL PROPERTIES OF GE:SB:TE TERNARY ALLOYS

The complex dielectric function of the three stoichiometric compositions GeSb4Te7, GeSb2Te4, and Ge2Sb2Te5 has been measured by spectroscopic ellipsometry in the energy range of 1.4–5.1 eV. Each composition was measured in the amorphous and in the two crystalline states, the metastable face-centered-cubic and the stable hexagonal phases. The complex dielectric function shows strong differences between the amorphous and the crystalline samples, these differences are responsible for the drastic contrast in the reflectivity spectra. The ellipsometric spectra are fitted with models which fulfilling the Kramers–Kronig relations. Thus the Forouhi–Bloomer model was used for the amorphous samples and the Lorentz harmonic oscillator model for the crystalline samples. The temperatures at which the phase transformations are achieved were determined by measuring the ac electrical resistance at 100 Hz while the samples were heated. The temperature derivative of the electrical conductivity shows two well defined peaks ...

Characterization of sol–gel glasses with different copper concentrations treated under oxidizing and reducing conditions

Abstract Using the sol–gel process SiO 2 coating and powdered samples doped with different concentrations of copper were prepared. Two sets of samples were made: one was thermally treated in air atmosphere, and the other, was annealed under a CO atmosphere. Both sets of samples were characterized using X-ray diffraction, optical transmission spectroscopy, photoacoustic absorption spectroscopy (PAS) in the visible and near infrared region and electron paramagnetic resonance (EPR). It is found that the chemical state of the copper depends on the type of treatment, amount of copper and the temperature of the treatment. The formations of CuO or Cu colloidal particles occur after the thermal treatments in air or in the CO atmosphere, respectively. In both cases, copper is found in the Cu 2+ state in interstitial positions and probably substituting the Si atoms in the SiO 2 matrix. The evolution of the optical absorption spectrum as function of the Cu content is a consequence of the changes in the composition and size of the colloidal particles. The optical and the PAS measurements are found to be complementary tools to elucidate the origin of the blue band in the absorption spectrum of these doped samples.

Photoluminescence studies in ZnxCd1−xTe single crystals

The crystalline quality of ZnxCd1−xTe single crystals prepared by a modified Bridgman method with 0≤x≤0.05 has been analyzed using photoluminescence. The spectrum of a typical sample is dominated by lines originating from the recombination of free and bound excitons. Lines due to free excitons in their ground and first excited states are observed in both the pure CdTe and the mixed crystals. Excitons bound to Cd vacancies are observed in the pure CdTe crystal but not in the mixed crystal. Weaker and broader features appearing at energies below the exciton emission range were associated with transitions involving free‐to‐bound and bound‐to‐bound levels. The origin of the various lines in the spectra was deduced from the detailed measurements of the dependence of the spectrum on temperature and excitation intensity.

Raman study of copper and iron oxide particles embedded in an SiO2 matrix

The structure of heat-treated sol–gel-made SiO2 samples containing copper or iron in various amounts was studied using Raman spectroscopy and x-ray diffraction. The treatments were in air in the temperature range 200–800 °C. Copper and iron concentrations were in the range 7–55 wt%. It was found that in the samples containing copper, the metal is mainly dissolved in the SiO2 network when the thermal treatment is at low temperatures. For higher annealing temperatures, first a copper silicate is formed and then most of the copper forms cupric oxide particles at temperatures of about 800 °C. In samples containing iron, the formation of particles with nanometric size of the γ-Fe2O3 (maghemite) phase is observed after annealing at 500 °C. At 800 °C the maghemite phase is transformed into the stable hematite (α-Fe2O3) phase. Copyright